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XR250R, Big Bore Kit


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For the 300-320cc big bore the cyl has to be bored,also case bored.It can be done in frame,with out taking apart lower end.The top of cyl also has to be machined,after sleeve is pressed in.

How is it possible to bore out a case without taking out the bottom end, I guess the rod is sticking out and everything? I personally don't know a whole lot about machining but wouldn't shavings go down into the bottom end? And also while it is in the frame!? I don't doubt you but wow, how? Seems like it would save a lot of $$$ not having to take the bottom end all apart.

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Ok the secret is out.I have made 8-. 300s.All still running.You mark it with the gaskit,then move the rod to one side.Then cover crank with plastic,than clay.Bore out half side,die grinder,or wood router,with metal cutter grinder.Does not need much.Do other side.Take out clay flush out with solvent.You dont need to worry about shavings as all 8 300s have been running over 10 years.Only mine after 10 years new piston,rings ,valves.Bore was still at 3.5 thou,but after a zillon miles,I did not want to chance it.

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I have an XR250R (2000) that i want to install a big bore kit. I ordered an 78m

sleeve and piston kit.

The Co. is telling me that that my cylinder and the sleeve will need to be bored. Is this correct?

You say you have a 78mm piston and a BB sleeve... correct? In that case you would have to bore the cylinder to accept the sleeve and sleeves always need to be bored to the piston. What the others have been saying though is that at 78mm on a 2000 you do not need a bigger sleeve. Put the sleeve on a shelf for another day and just have the one already in the cylinder bored to your 78mm piston. Save a lot of money.... Don't do the sleeve.

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Ok the secret is out.I have made 8-. 300s.All still running.You mark it with the gaskit,then move the rod to one side.Then cover crank with plastic,than clay.Bore out half side,die grinder,or wood router,with metal cutter grinder.Does not need much.Do other side.Take out clay flush out with solvent.You dont need to worry about shavings as all 8 300s have been running over 10 years.Only mine after 10 years new piston,rings ,valves.Bore was still at 3.5 thou,but after a zillon miles,I did not want to chance it.

I figured you had a trick or two up your sleeve :ride:. I don't suppose you have to have the type of tolerances as would be needed for a sleeve bored for the cylinder. ? Just as long as the sleeve can fit into the cases and be a snug enough without shifting, bolts should do good for that. ? ­čśĆ :smirk: So long as you can do some darn good careful catching of the shavings. Sounds like a lot of time but not as much time or money as gutting the cases! No guts no glory! I think a lot of people probably shy away from the 300 because of the extra expense of the case being bored, but oh man does a 300 sound good! :cheers:

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You mentioned that none of the guys have issues with their 300's if set up right. What is set up right? I am thining of doing this to my 250 but I would like to hear more about what your set ups are and maybe a little more indepth how that boring procedure was done. Thanks.

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